Hire Internet marketing specialist vs be marketing specialist yourself

This is fully depends on your product/service.

Lets imagine that the cost of one unit of your product/service costs $100.
On the Internet without marketing, you won't be able to sell it.

Then you find someone who suggest to advertise your product to 1000 people for $100.
And here your Internet marketing absolutely depends on how much your product was attractive to those 1000 people who saw it through your marketing channels.

Distinguish the marketing.
There's different ways to market your product on the Internet. Among them are SEO, posting (in forums/blogs/etc).